In the field of pulsed power, in which electricity is discharged at a high current and high voltage over a very short duration of time, the lack of a reliable switch has prevented widespread commercial implementation of pulsed power to nanopowder synthesis.
One of the techniques for synthesizing nanopowders involves the discharge of a high power pulsed arc between two electrodes, at least one of which is an ablating electrode composed of a precursor material. The high power pulsed arc is created by connecting the electrodes to a high-power, pulsed discharge power supply such as a capacitor bank. During the charging of the power supply, the electrodes are isolated from the charge voltage by means of an external high power, pulsed power switch. If the electrical standoff potential between the electrodes is less than the power supply charge voltage, then a high power, pulsed electrical discharge arc will be created across the electrodes when the external switch is closed.
The production of nanopowder by the above synthesis systems depends upon the reliability of the external pulsed power switch that is used. Prior pulsed power switches include the ignitron offered by Richardson Electronics of LaFox, Ill. as product number NL1488; solid-state switches such as those offered by International Rectifier of El Segundo, Calif. as product number ST3230C18RO, or by Powerex Inc. of Youngwood, Pa. as product number TO20443302; and generally available high power contact switches.
A typical ignitron, such as product number NL1488 offered by Richardson Electronics of LaFox, Ill., is a high power, vacuum mercury switch capable of 50 kV, 75 kA operation, and has an approximate life span of 100,000 operations. Further, the ignitron is comprised of a vacuum canister partially filled with a pool of mercury, two primary electrodes connected to the high-power pulsed-discharge power supply, and a secondary electrode. The secondary electrode and one of the primary electrodes must be connected to a second discharge power supply. In operation, the second discharge power supply is discharged to partially vaporize the mercury and thereby bring the primary electrodes into electrical contact. Thereafter, the high-power pulsed-discharge power supply is discharged to cause a high power pulsed arc between the primary electrodes. The disadvantages of an ignitron include limited life expectancies and a tendency toward catastrophic failures which may result in equipment destruction and mercury contamination.
Solid state switches such as an IGBT (insulating gate barrier transistor) or SCR (silicon controlled rectifier) switches are more reliable than ignitrons. However, solid state switches generally are more costly than the ignitron and have lower power ratings. A lower power rating translates to lower current capacity and/or voltage standoff. As a result, banks of switches in series and/or parallel switch configurations may be needed to switch a pulse comparable to that switched by an ignitron. While a lower power rating may be overcome by using multiple solid state switches in custom configurations, such configurations generally require cooling and sophisticated interconnects to reduce inductive and resistance effects, and ensure load balance. For high-power pulse discharges, a high resistance usually increases the amount of power required to generate a desired pulse, while a high inductance tends to cause an undesirable increase in pulse length.
High power contact or spark gap switches, although simplistic and less expensive than the ignitron or solid state switches, possess the disadvantage that they are generally destroyed after a single use in the high power, pulsed discharge environment described above.
Another disadvantage for systems with external high power switches is that the maximum gap between the electrodes necessary to achieve a desired standoff voltage is determined by the applied voltage. If the distance between electrodes is very small, wear on the production equipment occurs. Further, the electrodes are subjected to abnormal wear. This results in lower nanopowder yield and in equipment down time. If a higher applied voltage system is designed to increase the distance between the electrodes, the switch and production equipment components must be designed to accommodate the higher stand-off voltage. This results in higher costs and decreased system reliability.
One way that prior art systems have attempted to overcome the above disadvantages is through the use of a fuse wire between the electrodes. The gap between electrodes thereupon may be adjusted independent of the applied voltage. In operation, upon the external pulsed power switch being closed, the fuse wire explodes. As a result, a plasma is created that continues to provide an electrically conductive path to allow the pulsed power supply to discharge. The use of a fuse wire, however, requires that a new fuse wire be installed for each discharge. Further, unless the fuse wire is of the same composition as the precursor material of the electrodes, the fuse wire will contaminate the nanopowder which is produced.
The current invention is a nanopowder synthesis system which avoids the use of external high power switches and their attendant disadvantages, and endures repeated discharges (of the order of 107) between electrodes at a high repetition rate (≧1 Hz) without degradation. This is accomplished by effectively converting the electrodes into a high power switch by using a relatively low energy autofuser that provides a high voltage, high frequency current pulse between the electrodes to initiate a discharge from the main power supply. Although the general understanding in the relevant arts is that using the electrodes of a synthesis system as a switch is undesirable, because the tips of the electrodes are removed due to arc ablation that results in degradation of the switch, the current invention produces nanopowder from the ablated electrode material and indexes the electrodes toward each other to provide consistent operation. The ablation of the electrode tips thus becomes desirable, and initiation of the main power supply discharge across the electrodes occurs by way of a low energy autofuser. Thus, high average power discharges at high repetition rates can be performed reliably.